Disk drives (and other data-recording or playback devices) operate with a read/write head at a nominal distance above the disk, known as the fly height. Head touchdown occurs when the head effectively or substantially contacts the disk. Head touchdown detection is especially useful in disk drives which provide fly height adjustment.
Disk drives have detected head touchdown using a heater in the head. The disk drive supplies power to the heater so that the head thermally expands and protrudes towards the disk, thereby lowering the fly height. The power is supplied to the heater while the head is positioned over test tracks or other non-data-bearing areas of the disk and does not perform read or write operations. As more power is supplied to the heater, head touchdown is monitored. However, this approach is time consuming, often requiring a large number of disk revolutions (such as 100 disk revolutions) to accumulate sufficient data points.
Disk drives have also detected head touchdown by writing high-frequency patterns in servo fields and detecting the amplitude of such patterns. However, this approach requires new channel features and significant firmware changes and is subject to channel setting, channel noise and the like.
Disk drives include servo systems that position the head relative to the disk using a position error signal (PES) during track following, as is typical during read and write operations. The servo system reduces the impact of vibration or other external disturbances on the PES to avoid track misregistration. However, the servo system can also reduce the sensitivity of the PES to head touchdown. As a result, the servo system may be unable to distinguish or detect head touchdown (“false negative”), thereby damaging the head.
Accordingly, there is a need for a disk drive that detects head touchdown reliably, accurately and quickly.